TDMA communications systems typically comprise a large number of individual radio transceiver units, often originating with different manufacturers. In order to function properly, it is necessary that all units operating within the system function according to the same specifications. An example of this is the proposed Group Speciale Mobile (GSM) pan-European digital cellular system. This system would require that all components, such as transmitters and receivers, be manufactured to standard specifications measured by a common method.
In a TDMA system, digital information intended for a single receiver out of many is modulated onto a single frequency signal transmitted to all receivers. Multiple users of a single frequency signal are accommodated by allowing access to the signal by each individual user at different time intervals. In the GSM system, the timed access is divided into groups of eight time frames. Each time frame is an allotted time interval for one radio frequency (RF) burst containing digital information meant for an individual receiver. Obviously, in such a time dependent system, the timing of an analog measurement to determine conformance with a specification must be coordinated with the digital information in a RF burst in order to obtain meaningful measurement results. However, in a RF communications system, it is not always possible to have a trigger signal available to trigger the measurements. In these circumstances, it is necessary to trigger the measurement from the RF signal itself.
Existing measurement devices do not have the capacity of generating a trigger signal and determining its timing relationship to digital information in an RF burst from the RF burst itself. Instead, these prior devices require a direct, hard-wired connection to the TDMA system's frame clock to provide a trigger signal. Additionally, in order to coordinate the frame clock trigger signal with analog measurements of the RF burst, the user must provide the measurement device with a known timing relationship between the trigger signal and the RF burst on the TDMA system's signal. Thus, the user must know the location, in time, of the trigger signal relative to the RF burst to be measured. Some examples of these prior devices are the Hewlett-Packard (HP) 71125C, HP 71250C and HP 8591A with HP 85715A personality.
The present invention makes possible the triggering of analog measurements such as peak power of the RF signal, pulse on/off ratio of the RF burst or adjacent channel interference, from the RF burst itself. When triggered, these analog measurements are stored as a trace sample. The invention triggers the measurements by generating a trigger pulse when a rising RF level is detected in the TDMA signal. Although the illustrated embodiment of this invention uses RF level detection for triggering, edge detection of the rising edge of the RF burst can alternatively be implemented. Through digital signal processing (DSP) of the TDMA signal, the digital data transmitted by the radio under test is demodulated. The digital signal processor then searches for a predetermined midamble in the data from the RF burst that is used to synchronize to the data. Once synchronized, the DSP analyzer can determine the location of the trigger relative to the beginning of the data in the RF burst, to generate a timing reference. With the trigger pulse signal and the timing reference information, the invention is able to correctly identify which elements in the trace sample are to be used for analog measurements of the TDMA signal.
Additional features and advantages of the invention will be made apparent from the following description of the preferred embodiment, which proceeds with reference to the accompanying drawings.